The present invention relates to a method and a device for manufacturing a hologram recording medium and particularly relates to arts for manufacturing a hologram recording medium that is arranged to reproduce different original images when observed from different positions.
Holograms have come to be widely used in applications for preventing counterfeiting of cash vouchers and credit cards. Normally, a region onto which a hologram is to be recorded is set up in a portion of a medium to be subject to counterfeiting prevention, and a hologram of a three-dimensional image, etc., is recorded inside this region.
With many holograms that are currently utilized commercially, an original image is recorded onto a medium in the form of interference fringes by an optical method. That is, a method is employed in which an object that forms an original image is prepared and light from the object and a reference light are guided by a lens or other optical system to a recording surface, coated with a photosensitizing agent, to form interference fringes on the recording surface. Although this optical method requires an optical system of considerably high precision to obtain a clear image, it is the most direct method for obtaining a hologram and is the most widely practiced method in industry.
Meanwhile, methods for preparing a hologram by forming interference fringes on a recording surface by computation using a computer have come to be known recently, and a hologram prepared by such a method is generally referred to as a “computer generated hologram (CGH)” or simply as a “computer hologram.” A computer hologram is obtained by simulating an optical interference fringe generating process on a computer, and an entire process of generating an interference fringe pattern is carried out in the form of computation on the computer. Upon obtaining image data of an interference fringe pattern by such a computation, physical interference fringes are formed on an actual medium based on the image data. As a specific example, a method, with which image data of an interference fringe pattern prepared by a computer are provided to an electron beam printer and physical interference fringes are formed by scanning an electron beam across a medium, has been put to practical use.
With a hologram recording medium, an original image can be recorded three-dimensionally and the original image can be observed from different angles by changing the viewpoint position. Thus, a major characteristic of a hologram recording medium is that a three-dimensional image can be recorded on a flat surface. Also, recently, hologram recording media, with a further characteristic that a completely different original image is reproduced when observed from a different angle, are being utilized commercially. For example, Japanese Patent Laid-open Publication No. 2001-109362A discloses a method that employs a computer generating hologram method to manufacture a hologram recording medium with which different original images can be reproduced by changing the viewpoint position.
As mentioned above, methods for manufacturing a hologram recording medium, with which different original images can be reproduced when observed from different positions, are already as known as conventional arts. However, because the basic principle of the conventional methods is to set up a plurality of regions on a hologram recording surface and record a different original image on each individual region, there is the problem that the reproduced images are lowered in resolution.
For example, the abovementioned Patent Document discloses a method in which a hologram recording surface is partitioned into a plurality of strip-like regions, each strip-like region is associated with one original image among a plurality of mutually different original images, and on a single strip-like region, only the one original image that is associated with the strip-like region is recorded. Specifically, in a case where three original images are to be recorded, a recording method is employed in which a first original image is recorded on a 1st, 4th, 7th, 10th strip-like regions, etc., a second original image is recorded on a 2nd, 5th, 8th, 11th strip-like regions, etc., and a third original image is recorded on a 3rd, 6th, 9th, 12th strip-like regions, etc. In this case, each of the three original images is recorded in the form of interference fringes and by differing the direction of the reference light according to each original image in this process, a specific original image is made to be reproduced upon observation from a specific position.
However with the above example, because, for example, the first original image is recorded only on the 1st, 4th, 7th, 10th strip-like regions, etc., and information on the first original image are left out from the 2nd, 3rd, 5th, 6th, 8th, 9th, 11th, 12th strip-like regions, etc., the resolution of the reproduced image is reduced to ⅓ that of the original. Thus, as long as the principle of recording a different original image on each individual region is employed, the problem of lowering the resolution of the reproduced image occurs.